Carbon Filament Light BulbJune 18, 2008 8:20 pm Projects
Here’s a genuine antique carbon filament light bulb. It’s 375 watts and was originally meant for 110 volts (currently in the USA we use 120 volts AC). In the photo, the bulb is running from 50 volts.
Perhaps someone out there has more information on the age of this bulb. I think it’s around 60-70 years old. It’s not older since it doesn’t have the glass seal on the top of the bulb.
This graph shows the resistance of the filament in two types of light bulbs. The blue curve shows that a carbon filament decreases in resistance as the bulb heats up, and the pink curve shows that a tungsten filament bulb increases in resistance as it heats up.
Thus, carbon filament bulbs have a negative temperature coefficient and tungsten filament light bulbs have a positive temperature coefficient.
Tungsten is the filament material most commonly used in household light bulbs.
Incidentally the curve for the carbon filament bulb stops short at 90V because I don’t want to damage it. It runs very, very hot!